Short-circuit evaluation

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Task
Short-circuit evaluation
You are encouraged to solve this task according to the task description, using any language you may know.
Control Structures

These are examples of control structures. You may also be interested in:

Assume functions a and b return boolean values, and further, the execution of function b takes considerable resources without side effects, and is to be minimised.

If we needed to compute the conjunction (and):

x = a() and b()

Then it would be best to not compute the value of b() if the value of a() is computed as false, as the value of x can then only ever be false.

Similarly, if we needed to compute the disjunction (or):

y = a() or b()

Then it would be best to not compute the value of b() if the value of a() is computed as true, as the value of y can then only ever be true.

Some languages will stop further computation of boolean equations as soon as the result is known, so-called short-circuit evaluation of boolean expressions

Task Description

The task is to create two functions named a and b, that take and return the same boolean value. The functions should also print their name whenever they are called. Calculate and assign the values of the following equations to a variable in such a way that function b is only called when necessary:

x = a(i) and b(j)
y = a(i) or b(j)

If the language does not have short-circuit evaluation, this might be achieved with nested if statements.

Contents

[edit] Ada

Ada has built-in short-circuit operations and then and or else:

with Ada.Text_IO;  use Ada.Text_IO;
 
procedure Test_Short_Circuit is
function A (Value : Boolean) return Boolean is
begin
Put (" A=" & Boolean'Image (Value));
return Value;
end A;
function B (Value : Boolean) return Boolean is
begin
Put (" B=" & Boolean'Image (Value));
return Value;
end B;
begin
for I in Boolean'Range loop
for J in Boolean'Range loop
Put (" (A and then B)=" & Boolean'Image (A (I) and then B (J)));
New_Line;
end loop;
end loop;
for I in Boolean'Range loop
for J in Boolean'Range loop
Put (" (A or else B)=" & Boolean'Image (A (I) or else B (J)));
New_Line;
end loop;
end loop;
end Test_Short_Circuit;
Sample output:
 A=FALSE (A and then B)=FALSE
 A=FALSE (A and then B)=FALSE
 A=TRUE B=FALSE (A and then B)=FALSE
 A=TRUE B=TRUE (A and then B)=TRUE
 A=FALSE B=FALSE (A or else B)=FALSE
 A=FALSE B=TRUE (A or else B)=TRUE
 A=TRUE (A or else B)=TRUE
 A=TRUE (A or else B)=TRUE

[edit] ALGOL 68

[edit] With Standard

Works with: ALGOL 68 version Revision 1 - no extensions to language used
Works with: ALGOL 68G version Any - tested with release 1.18.0-9h.tiny
Works with: ELLA ALGOL 68 version Any (with appropriate job cards) - tested with release 1.8-8d

Note: The "brief" conditional clause ( ~ | ~ | ~ ) is a the standard's shorthand for enforcing short-circuit evaluation. Moreover, the coder is able to define their own proc[edures] and op[erators] that implement short-circuit evaluation by using Algol68's proceduring.

PRIO ORELSE = 2, ANDTHEN = 3; # user defined operators #
OP ORELSE = (BOOL a, PROC BOOL b)BOOL: ( a | a | b ),
ANDTHEN = (BOOL a, PROC BOOL b)BOOL: ( a | b | a );
 
# user defined Short-circuit_evaluation procedures #
PROC or else = (BOOL a, PROC BOOL b)BOOL: ( a | a | b ),
and then = (BOOL a, PROC BOOL b)BOOL: ( a | b | a );
 
test:(
 
PROC a = (BOOL a)BOOL: ( print(("a=",a,", ")); a),
b = (BOOL b)BOOL: ( print(("b=",b,", ")); b);
 
CO
# Valid for Algol 68 Rev0: using "user defined" operators #
# Note: here BOOL is being automatically "procedured" to PROC BOOL #
print(("T ORELSE F = ", a(TRUE) ORELSE b(FALSE), new line));
print(("F ANDTHEN T = ", a(FALSE) ANDTHEN b(TRUE), new line));
 
print(("or else(T, F) = ", or else(a(TRUE), b(FALSE)), new line));
print(("and then(F, T) = ", and then(a(FALSE), b(TRUE)), new line));
END CO
 
# Valid for Algol68 Rev1: using "user defined" operators #
# Note: BOOL must be manually "procedured" to PROC BOOL #
print(("T ORELSE F = ", a(TRUE) ORELSE (BOOL:b(FALSE)), new line));
print(("T ORELSE T = ", a(TRUE) ORELSE (BOOL:b(TRUE)), new line));
 
print(("F ANDTHEN F = ", a(FALSE) ANDTHEN (BOOL:b(FALSE)), new line));
print(("F ANDTHEN T = ", a(FALSE) ANDTHEN (BOOL:b(TRUE)), new line));
 
print(("F ORELSE F = ", a(FALSE) ORELSE (BOOL:b(FALSE)), new line));
print(("F ORELSE T = ", a(FALSE) ORELSE (BOOL:b(TRUE)), new line));
 
print(("T ANDTHEN F = ", a(TRUE) ANDTHEN (BOOL:b(FALSE)), new line));
print(("T ANDTHEN T = ", a(TRUE) ANDTHEN (BOOL:b(TRUE)), new line))
 
)
Output:
a=T, T ORELSE F = T
a=T, T ORELSE T = T
a=F, F ANDTHEN F = F
a=F, F ANDTHEN T = F
a=F, b=F, F ORELSE F = F
a=F, b=T, F ORELSE T = T
a=T, b=F, T ANDTHEN F = F
a=T, b=T, T ANDTHEN T = T

[edit] With Extensions

Works with: ALGOL 68G version Any - tested with release 1.18.0-9h.tiny
Works with: ELLA ALGOL 68 version Any (with appropriate job cards) - tested with release 1.8-8d
test:(
 
PROC a = (BOOL a)BOOL: ( print(("a=",a,", ")); a),
b = (BOOL b)BOOL: ( print(("b=",b,", ")); b);
 
# Valid for Algol 68G and 68RS using non standard operators #
print(("T OREL F = ", a(TRUE) OREL b(FALSE), new line));
print(("T OREL T = ", a(TRUE) OREL b(TRUE), new line));
 
print(("F ANDTH F = ", a(FALSE) ANDTH b(FALSE), new line));
print(("F ANDTH T = ", a(FALSE) ANDTH b(TRUE), new line));
 
print(("F OREL F = ", a(FALSE) OREL b(FALSE), new line));
print(("F OREL T = ", a(FALSE) OREL b(TRUE), new line));
 
print(("T ANDTH F = ", a(TRUE) ANDTH b(FALSE), new line));
print(("T ANDTH T = ", a(TRUE) ANDTH b(TRUE), new line))
 
CO;
# Valid for Algol 68G and 68C using non standard operators #
print(("T ORF F = ", a(TRUE) ORF b(FALSE), new line));
print(("F ANDF T = ", a(FALSE) ANDF b(TRUE), new line))
END CO
 
)
Output:
a=T, T OREL F = T
a=T, T OREL T = T
a=F, F ANDTH F = F
a=F, F ANDTH T = F
a=F, b=F, F OREL F = F
a=F, b=T, F OREL T = T
a=T, b=F, T ANDTH F = F
a=T, b=T, T ANDTH T = T

[edit] AutoHotkey

In AutoHotkey, the boolean operators, and, or, and ternaries, short-circuit:

i = 1
j = 1
x := a(i) and b(j)
y := a(i) or b(j)
 
a(p)
{
MsgBox, a() was called with the parameter "%p%".
Return, p
}
 
b(p)
{
MsgBox, b() was called with the parameter "%p%".
Return, p
}

[edit] AWK

Short-circuit evalation is done in logical AND (&&) and logical OR (||) operators:

#!/usr/bin/awk -f 
BEGIN {
print (a(1) && b(1))
print (a(1) || b(1))
print (a(0) && b(1))
print (a(0) || b(1))
}
 
 
function a(x) {
print " x:"x
return x
}
function b(y) {
print " y:"y
return y
}
Output:
  x:1
  y:1
1
  x:1
1
  x:0
0
  x:0
  y:1
1

[edit] BBC BASIC

Short-circuit operators aren't implemented directly, but short-circuit AND can be simulated using cascaded IFs. Short-circuit OR can be converted into a short-circuit AND using De Morgan's laws.

      REM TRUE is represented as -1, FALSE as 0
FOR i% = TRUE TO FALSE
FOR j% = TRUE TO FALSE
PRINT "For x=a(";FNboolstring(i%);") AND b(";FNboolstring(j%);")"
x% = FALSE
REM Short-circuit AND can be simulated by cascaded IFs:
IF FNa(i%) IF FNb(j%) THEN x%=TRUE
PRINT "x is ";FNboolstring(x%)
PRINT
PRINT "For y=a(";FNboolstring(i%);") OR b(";FNboolstring(j%);")"
y% = FALSE
REM Short-circuit OR can be simulated by De Morgan's laws:
IF NOTFNa(i%) IF NOTFNb(j%) ELSE y%=TRUE : REM Note ELSE without THEN
PRINT "y is ";FNboolstring(y%)
PRINT
NEXT:NEXT
END
 
DEFFNa(bool%)
PRINT "Function A used; ";
=bool%
 
DEFFNb(bool%)
PRINT "Function B used; ";
=bool%
 
DEFFNboolstring(bool%)
IF bool%=0 THEN ="FALSE" ELSE="TRUE"

This gives the results shown below:

For x=a(TRUE) AND b(TRUE)
Function A used; Function B used; x is TRUE

For y=a(TRUE) OR b(TRUE)
Function A used; y is TRUE

For x=a(TRUE) AND b(FALSE)
Function A used; Function B used; x is FALSE

For y=a(TRUE) OR b(FALSE)
Function A used; y is TRUE

For x=a(FALSE) AND b(TRUE)
Function A used; x is FALSE

For y=a(FALSE) OR b(TRUE)
Function A used; Function B used; y is TRUE

For x=a(FALSE) AND b(FALSE)
Function A used; x is FALSE

For y=a(FALSE) OR b(FALSE)
Function A used; Function B used; y is FALSE

[edit] Bracmat

Bracmat has no booleans. The closest thing is the success or failure of an expression. A function is not called if the argument fails, so we have to use a trick to pass 'failure' to a function. Here it is accomplished by an extra level of indirection: two == in the definition of 'false' (and 'true', for symmetry) and two !! when evaluating the argument in the functions a and b. The backtick is another hack. This prefix tells Bracmat to look the other way if the backticked expression fails and to continue as if the expression succeeded. A neater way is to introduce an extra OR operator. That solution would have obscured the core of the current task. Short-circuit evaluation is heavily used in Bracmat code. Although not required, it is a good habit to exclusively use AND (&) and OR (|) operators to separate expressions, as the code below exemplifies.

( (a=.out$"I'm a"&!!arg)
& (b=.out$"I'm b"&!!arg)
& (false==~)
& (true==)
& !false !true:?outer
& whl
' ( !outer:%?x ?outer
& !false !true:?inner
& whl
' ( !inner:%?y ?inner
& out
$ ( Testing
(!!x&true|false)
AND
(!!y&true|false)
)
& `(a$!x&b$!y)
& out
$ ( Testing
(!!x&true|false)
OR
(!!y&true|false)
)
& `(a$!x|b$!y)
)
)
& done
);
 

Output:

Testing false AND false
I'm a
Testing false OR false
I'm a
I'm b
Testing false AND true
I'm a
Testing false OR true
I'm a
I'm b
Testing true AND false
I'm a
I'm b
Testing true OR false
I'm a
Testing true AND true
I'm a
I'm b
Testing true OR true
I'm a

[edit] C

Boolean operators && and || are shortcircuit operators.

#include <stdio.h>
#include <stdbool.h>
 
bool a(bool in)
{
printf("I am a\n");
return in;
}
 
bool b(bool in)
{
printf("I am b\n");
return in;
}
 
#define TEST(X,Y,O) \
do { \
x = a(X) O b(Y); \
printf(#X " " #O " " #Y " = %s\n\n", x ? "true" : "false"); \
} while(false);

 
int main()
{
bool x;
 
TEST(false, true, &&); // b is not evaluated
TEST(true, false, ||); // b is not evaluated
TEST(true, false, &&); // b is evaluated
TEST(false, false, ||); // b is evaluated
 
return 0;
}

[edit] C++

Just like C, boolean operators && and || are shortcircuit operators.

#include <iostream>
 
bool a(bool in)
{
std::cout << "a" << std::endl;
return in;
}
 
bool b(bool in)
{
std::cout << "b" << std::endl;
return in;
}
 
void test(bool i, bool j) {
std::cout << std::boolalpha << i << " and " << j << " = " << (a(i) && b(j)) << std::endl;
std::cout << std::boolalpha << i << " or " << j << " = " << (a(i) || b(j)) << std::endl;
}
 
int main()
{
test(false, false);
test(false, true);
test(true, false);
test(true, true);
return 0;
}
Output:
a 
false and false = false
a 
b 
false or false = false
a 
false and true = false
a 
b 
false or true = true
a 
b 
true and false = false
a 
true or false = true
a 
b 
true and true = true
a 
true or true = true

[edit] C#

using System;
 
class Program
{
static bool a(bool value)
{
Console.WriteLine("a");
return value;
}
 
static bool b(bool value)
{
Console.WriteLine("b");
return value;
}
 
static void Main()
{
foreach (var i in new[] { false, true })
{
foreach (var j in new[] { false, true })
{
Console.WriteLine("{0} and {1} = {2}", i, j, a(i) && b(j));
Console.WriteLine();
Console.WriteLine("{0} or {1} = {2}", i, j, a(i) || b(j));
Console.WriteLine();
}
}
}
}
Output:
a
False and False = False
 
a
b
False or False = False
 
a
False and True = False
 
a
b
False or True = True
 
a
b
True and False = False
 
a
True or False = True
 
a
b
True and True = True
 
a
True or True = True

[edit] Clojure

The print/println stuff in the doseq is kinda gross, but if you include them all in a single print, then the function traces are printed before the rest (since it has to evaluate them before calling print).

(letfn [(a [bool] (print "(a)") bool)   
(b [bool] (print "(b)") bool)]
(doseq [i [true false] j [true false]]
(print i "OR" j "= ")
(println (or (a i) (b j)))
(print i "AND" j " = ")
(println (and (a i) (b j)))))
Output:
true OR true = (a)true       
true AND true  = (a)(b)true  
true OR false = (a)true      
true AND false  = (a)(b)false
false OR true = (a)(b)true   
false AND true  = (a)false   
false OR false = (a)(b)false 
false AND false  = (a)false

[edit] Common Lisp

(defun a (F)
(print 'a)
F )
 
(defun b (F)
(print 'b)
F )
 
(dolist (x '((nil nil) (nil T) (T T) (T nil)))
(format t "~%(and ~S)" x)
(and (a (car x)) (b (car(cdr x))))
(format t "~%(or ~S)" x)
(or (a (car x)) (b (car(cdr x)))))
Output:
(and (NIL NIL))
A 
(or (NIL NIL))
A 
B 
(and (NIL T))
A 
(or (NIL T))
A 
B 
(and (T T))
A 
B 
(or (T T))
A 
(and (T NIL))
A 
B 
(or (T NIL))
A

[edit] D

Translation of: Python
import std.stdio, std.algorithm;
 
T a(T)(T answer) {
writefln(" # Called function a(%s) -> %s", answer, answer);
return answer;
}
 
T b(T)(T answer) {
writefln(" # Called function b(%s) -> %s", answer, answer);
return answer;
}
 
void main() {
foreach (immutable x, immutable y;
[false, true].cartesianProduct([false, true])) {
writeln("\nCalculating: r1 = a(x) && b(y)");
immutable r1 = a(x) && b(y);
writeln("Calculating: r2 = a(x) || b(y)");
immutable r2 = a(x) || b(y);
}
}
Output:
Calculating: r1 = a(x) && b(y)
  # Called function a(false) -> false
Calculating: r2 = a(x) || b(y)
  # Called function a(false) -> false
  # Called function b(false) -> false

Calculating: r1 = a(x) && b(y)
  # Called function a(true) -> true
  # Called function b(false) -> false
Calculating: r2 = a(x) || b(y)
  # Called function a(true) -> true

Calculating: r1 = a(x) && b(y)
  # Called function a(false) -> false
Calculating: r2 = a(x) || b(y)
  # Called function a(false) -> false
  # Called function b(true) -> true

Calculating: r1 = a(x) && b(y)
  # Called function a(true) -> true
  # Called function b(true) -> true
Calculating: r2 = a(x) || b(y)
  # Called function a(true) -> true

[edit] Delphi

Delphi supports short circuit evaluation by default. It can be turned off using the {$BOOLEVAL OFF} compiler directive.

program ShortCircuitEvaluation;
 
{$APPTYPE CONSOLE}
 
uses SysUtils;
 
function A(aValue: Boolean): Boolean;
begin
Writeln('a');
Result := aValue;
end;
 
function B(aValue: Boolean): Boolean;
begin
Writeln('b');
Result := aValue;
end;
 
var
i, j: Boolean;
begin
for i in [False, True] do
begin
for j in [False, True] do
begin
Writeln(Format('%s and %s = %s', [BoolToStr(i, True), BoolToStr(j, True), BoolToStr(A(i) and B(j), True)]));
Writeln;
Writeln(Format('%s or %s = %s', [BoolToStr(i, True), BoolToStr(j, True), BoolToStr(A(i) or B(j), True)]));
Writeln;
end;
end;
end.

[edit] E

E defines && and || in the usual short-circuiting fashion.

def a(v) { println("a"); return v }
def b(v) { println("b"); return v }
 
def x := a(i) && b(j)
def y := b(i) || b(j)

Unusually, E is an expression-oriented language, and variable bindings (which are expressions) are in scope until the end of the nearest enclosing { ... } block. The combination of these features means that some semantics must be given to a binding occurring inside of a short-circuited alternative.

def x := a(i) && (def funky := b(j))

The choice we make is that funky is ordinary if the right-side expression was evaluated, and otherwise is ruined; attempts to access the variable give an error.

[edit] Erlang

 
-module( short_circuit_evaluation ).
 
-export( [task/0] ).
 
task() ->
[task_helper(X, Y) || X <- [true, false], Y <- [true, false]].
 
 
 
a( Boolean ) ->
io:fwrite( " a ~p~n", [Boolean] ),
Boolean.
 
b( Boolean ) ->
io:fwrite( " b ~p~n", [Boolean] ),
Boolean.
 
task_helper( Boolean1, Boolean2 ) ->
io:fwrite( "~p andalso ~p~n", [Boolean1, Boolean2] ),
io:fwrite( "=> ~p~n", [a(Boolean1) andalso b(Boolean2)] ),
io:fwrite( "~p orelse ~p~n", [Boolean1, Boolean2] ),
io:fwrite( "=> ~p~n", [a(Boolean1) orelse b(Boolean2)] ).
 
Output:
15> short_circuit_evaluation:task().
true andalso true
 a true
 b true
=> true
true orelse true
 a true
=> true
true andalso false
 a true
 b false
=> false
true orelse false
 a true
=> true
false andalso true
 a false
=> false
false orelse true
 a false
 b true
=> true
false andalso false
 a false
=> false
false orelse false
 a false
 b false
=> false


[edit] F#

let a (x : bool) = printf "(a)"; x
let b (x : bool) = printf "(b)"; x
 
[for x in [true; false] do for y in [true; false] do yield (x, y)]
|> List.iter (fun (x, y) ->
printfn "%b AND %b = %b" x y ((a x) && (b y))
printfn "%b OR %b = %b" x y ((a x) || (b y)))

Output

(a)(b)true AND true = true
(a)true OR true = true
(a)(b)true AND false = false
(a)true OR false = true
(a)false AND true = false
(a)(b)false OR true = true
(a)false AND false = false
(a)(b)false OR false = false

[edit] Fantom

class Main
{
static Bool a (Bool value)
{
echo ("in a")
return value
}
 
static Bool b (Bool value)
{
echo ("in b")
return value
}
 
public static Void main ()
{
[false,true].each |i|
{
[false,true].each |j|
{
Bool result := a(i) && b(j)
echo ("a($i) && b($j): " + result)
result = a(i) || b(j)
echo ("a($i) || b($j): " + result)
}
}
}
}
Output:
in a
a(false) && b(false): false
in a
in b
a(false) || b(false): false
in a
a(false) && b(true): false
in a
in b
a(false) || b(true): true
in a
in b
a(true) && b(false): false
in a
a(true) || b(false): true
in a
in b
a(true) && b(true): true
in a
a(true) || b(true): true

[edit] Forth

\ Short-circuit evaluation definitions from Wil Baden, with minor name changes
: ENDIF postpone THEN ; immediate
 
: COND 0 ; immediate
: ENDIFS BEGIN DUP WHILE postpone ENDIF REPEAT DROP ; immediate
: ORELSE s" ?DUP 0= IF" evaluate ; immediate
: ANDIF s" DUP IF DROP" evaluate ; immediate
 
: .bool IF ." true " ELSE ." false " THEN ;
: A ." A=" DUP .bool ;
: B ." B=" DUP .bool ;
 
: test
CR
1 -1 DO 1 -1 DO
COND I A ANDIF J B ENDIFS ." ANDIF=" .bool CR
COND I A ORELSE J B ENDIFS ." ORELSE=" .bool CR
LOOP LOOP ;
 
\ An alternative based on explicitly short-circuiting conditionals, Dave Keenan
: END-PRIOR-IF 1 CS-ROLL postpone ENDIF ; immediate
 
: test
CR
1 -1 DO 1 -1 DO
I A IF J B IF 1 ELSE END-PRIOR-IF 0 ENDIF ." ANDIF=" .bool CR
I A 0= IF J B IF END-PRIOR-IF 1 ELSE 0 ENDIF ." ORELSE=" .bool CR
LOOP LOOP ;
Output:
A=true  B=true  ANDIF=true
A=true  ORELSE=true
A=false ANDIF=false
A=false B=true  ORELSE=true
A=true  B=false ANDIF=false
A=true  ORELSE=true
A=false ANDIF=false
A=false B=false ORELSE=false

[edit] Fortran

Works with: Fortran version 90 and later

Using an IF .. THEN .. ELSE construct

program Short_Circuit_Eval
implicit none
 
logical :: x, y
logical, dimension(2) :: l = (/ .false., .true. /)
integer :: i, j
 
do i = 1, 2
do j = 1, 2
write(*, "(a,l1,a,l1,a)") "Calculating x = a(", l(i), ") and b(", l(j), ")"
! a AND b
x = a(l(i))
if(x) then
x = b(l(j))
write(*, "(a,l1)") "x = ", x
else
write(*, "(a,l1)") "x = ", x
end if
 
write(*,*)
write(*, "(a,l1,a,l1,a)") "Calculating y = a(", l(i), ") or b(", l(j), ")"
! a OR b
y = a(l(i))
if(y) then
write(*, "(a,l1)") "y = ", y
else
y = b(l(j))
write(*, "(a,l1)") "y = ", y
end if
write(*,*)
end do
end do
 
contains
 
function a(value)
logical :: a
logical, intent(in) :: value
 
a = value
write(*, "(a,l1,a)") "Called function a(", value, ")"
end function
 
function b(value)
logical :: b
logical, intent(in) :: value
 
b = value
write(*, "(a,l1,a)") "Called function b(", value, ")"
end function
end program
Output:
Calculating x = a(F) and b(F)
Called function a(F)
x = F
 
Calculating y = a(F) or b(F)
Called function a(F)
Called function b(F)
y = F
 
Calculating x = a(F) and b(T)
Called function a(F)
x = F
 
Calculating y = a(F) or b(T)
Called function a(F)
Called function b(T)
y = T
 
Calculating x = a(T) and b(F)
Called function a(T)
Called function b(F)
x = F
 
Calculating y = a(T) or b(F)
Called function a(T)
y = T
 
Calculating x = a(T) and b(T)
Called function a(T)
Called function b(T)
x = T
 
Calculating y = a(T) or b(T)
Called function a(T)
y = T

[edit] Go

Short circuit operators are && and ||.

package main
 
import "fmt"
 
func a(v bool) bool {
fmt.Print("a")
return v
}
 
func b(v bool) bool {
fmt.Print("b")
return v
}
 
func test(i, j bool) {
fmt.Printf("Testing a(%t) && b(%t)\n", i, j)
fmt.Print("Trace: ")
fmt.Println("\nResult:", a(i) && b(j))
 
fmt.Printf("Testing a(%t) || b(%t)\n", i, j)
fmt.Print("Trace: ")
fmt.Println("\nResult:", a(i) || b(j))
 
fmt.Println("")
}
 
func main() {
test(false, false)
test(false, true)
test(true, false)
test(true, true)
}
Output:
Testing a(false) && b(false)
Trace:  a
Result: false
Testing a(false) || b(false)
Trace:  ab
Result: false

Testing a(false) && b(true)
Trace:  a
Result: false
Testing a(false) || b(true)
Trace:  ab
Result: true

Testing a(true) && b(false)
Trace:  ab
Result: false
Testing a(true) || b(false)
Trace:  a
Result: true

Testing a(true) && b(true)
Trace:  ab
Result: true
Testing a(true) || b(true)
Trace:  a
Result: true

[edit] Groovy

Like all C-based languages (of which I am aware), Groovy short-circuits the logical and (&&) and logical or (||) operations, but not the bitwise and (&) and bitwise or (|) operations.

def f = { println '  AHA!'; it instanceof String }
def g = { printf ('%5d ', it); it > 50 }
 
println 'bitwise'
assert g(100) & f('sss')
assert g(2) | f('sss')
assert ! (g(1) & f('sss'))
assert g(200) | f('sss')
 
println '''
logical'''

assert g(100) && f('sss')
assert g(2) || f('sss')
assert ! (g(1) && f('sss'))
assert g(200) || f('sss')
Output:
bitwise
  100   AHA!
    2   AHA!
    1   AHA!
  200   AHA!

logical
  100   AHA!
    2   AHA!
    1   200

[edit] Haskell

Lazy evaluation makes it possible for user-defined functions to be short-circuited. An expression will not be evaluated as long as it is not pattern matched:

module ShortCircuit where
 
import Prelude hiding ((&&), (||))
import Debug.Trace
 
False && _ = False
True && False = False
_ && _ = True
 
True || _ = True
False || True = True
_ || _ = False
 
a p = trace ("<a " ++ show p ++ ">") p
b p = trace ("<b " ++ show p ++ ">") p
 
main = mapM_ print ( [ a p || b q | p <- [False, True], q <- [False, True] ]
++ [ a p && b q | p <- [False, True], q <- [False, True] ])
Output:
<a False>
<b False>
False
<a False>
<b True>
True
<a True>
True
<a True>
True
<a False>
False
<a False>
False
<a True>
<b False>
False
<a True>
<b True>
True

One can force the right-hand arguemnt to be evaluated first be using the alternate definitions:

_     && False = False
False && True = False
_ && _ = True
 
_ || True = True
True || False = True
_ || _ = False
Output:
<b False>
<a False>
False
<b True>
True
<b False>
<a True>
True
<b True>
True
<b False>
False
<b True>
<a False>
False
<b False>
False
<b True>
<a True>
True

The order of evaluation (in this case the original order again) can be seen in a more explicit form by desugaring the pattern matching:

p && q = case p of
False -> False
_ -> case q of
False -> False
_ -> True
 
p || q = case p of
True -> True
_ -> case q of
True -> True
_ -> False

[edit] Icon and Unicon

The entire concept of using 'boolean' values for logic control runs counter to the philosophy of Icon. Instead Icon has success (something that returns a result) and failure which is really a signal. The concept is similar to that used in SNOBOL4 and Lisp and far more potent than passing around and testing booleans. There is no way to pass around a 'false' value in that sense. Icon does have facilities for dealing with bits inside integers but these would not normally be used for control purposes. Because failure is a signal control is always evaluated in a short-circuit manner. One consequence of this is that an expression "i < j" doesn't return a boolean value, instead it returns the value of j. While this may seem odd at first it allows for elegant expressions like "i < j < k". Another benefit is that there is no need for programmers to devote effort to staying inside the bounds of any data type. For instance, if you loop and iterate beyond bounds the expression simply fails and the loop ends.

While this task could be written literally, it would be more beneficial to show how an Icon programmer would approach the same problem. Icon extends the idea short circuit evaluation with the ability for expressions to generate alternate results only if needed. For more information see Failure is an option, Everything Returns a Value Except when it Doesn't, and Goal-Directed Evaluation and Generators. Consequently some small liberties will be taken with this task:

  • Since any result means an expression succeeded and is hence true, we can use any value. In this example our choice will be determined by how we deal with 'false'.
  • The inability to pass a 'false' value is a challenge. At first glance we might try &null, similar to Lisp, but there is no canonical true. Also &null produces a result, so strictly speaking it could be 'true' as well. A good example of this is that an expression like " not expr " returns null if 'expr' fails.
  • For this example we will define two procedures 'true' and 'false'. Because Icon treats procedures as a data type we can assign them and invoke them indirectly via the variable name they are assigned to. We can write " i := true " and later invoke 'true' via " i() ".
  • Rather than have the tasks print their own name, we will just utilize built-in tracing which will be more informative.

This use of procedures as values is somewhat contrived but serves us well for demonstration purposes. In practice this approach would be strained since failure results aren't re-captured as values (and can't easily be).

procedure main()
&trace := -1 # ensures functions print their names
 
every (i := false | true ) & ( j := false | true) do {
write("i,j := ",image(i),", ",image(j))
write("i & j:")
x := i() & j() # invoke true/false
write("i | j:")
y := i() | j() # invoke true/false
}
end
 
procedure true() #: succeeds always (returning null)
return
end
 
procedure false() #: fails always
fail # for clarity but not needed as running into end has the same effect
end
Sample output for a single case:
i,j := procedure true, procedure false
i & j:
Shortcircuit.icn:    8  | true()
Shortcircuit.icn:   16  | true returned &null
Shortcircuit.icn:    8  | false()
Shortcircuit.icn:   20  | false failed
i | j:
Shortcircuit.icn:   10  | true()
Shortcircuit.icn:   16  | true returned &null
i,j := procedure true, procedure true

[edit] J

See the J wiki entry on short circuit booleans.

labeled=:1 :'[ smoutput@,&":~&m'
A=: 'A ' labeled
B=: 'B ' labeled
and=: ^:
or=: 2 :'u^:(-.@v)'
Example:
   (A and B) 1
B 1
A 1
1
(A and B) 0
B 0
0
(A or B) 1
B 1
1
(A or B) 0
B 0
A 0
0

Note that J evaluates right-to-left.

Note also that both functions take the same argument (which might make this less than ideal for some purposes, but trying micromanage flow of control is usually counter-productive in J in much the way that global values can be counter-productive in an object oriented environment. When you are processing a large set of array data, flow of control can only make sense when it is relevant to all of the data being processed -- if you want to manage flow of control which is not relevant to the entire set of data being processed you might artificially reduce the amount of data being processed, along the lines of an SQL cursor).

[edit] Java

In Java the boolean operators && and || are short circuit operators. The eager operator counterparts are & and |.

public class ShortCirc {
public static void main(String[] args){
System.out.println("F and F = " + (a(false) && b(false)) + "\n");
System.out.println("F or F = " + (a(false) || b(false)) + "\n");
 
System.out.println("F and T = " + (a(false) && b(true)) + "\n");
System.out.println("F or T = " + (a(false) || b(true)) + "\n");
 
System.out.println("T and F = " + (a(true) && b(false)) + "\n");
System.out.println("T or F = " + (a(true) || b(false)) + "\n");
 
System.out.println("T and T = " + (a(true) && b(true)) + "\n");
System.out.println("T or T = " + (a(true) || b(true)) + "\n");
}
 
public static boolean a(boolean a){
System.out.println("a");
return a;
}
 
public static boolean b(boolean b){
System.out.println("b");
return b;
}
}
Output:
a
F and F = false

a
b
F or F = false

a
F and T = false

a
b
F or T = true

a
b
T and F = false

a
T or F = true

a
b
T and T = true

a
T or T = true

[edit] Julia

Julia does have short-circuit evaluation, which works just as you expect it to:

a(x) = (println("\t# Called a($x)"); return x)
b(x) = (println("\t# Called b($x)"); return x)
 
for i in [true,false], j in [true, false]
println("\nCalculating: x = a($i) && b($j)"); x = a(i) && b(j)
println("\tResult: x = $x")
println("\nCalculating: y = a($i) || b($j)"); y = a(i) || b(j)
println("\tResult: y = $y")
end
Output:
Calculating: x = a(true) && b(true)
	# Called a(true)
	# Called b(true)
	Result: x = true

Calculating: y = a(true) || b(true)
	# Called a(true)
	Result: y = true

Calculating: x = a(true) && b(false)
	# Called a(true)
	# Called b(false)
	Result: x = false

Calculating: y = a(true) || b(false)
	# Called a(true)
	Result: y = true

Calculating: x = a(false) && b(true)
	# Called a(false)
	Result: x = false

Calculating: y = a(false) || b(true)
	# Called a(false)
	# Called b(true)
	Result: y = true

Calculating: x = a(false) && b(false)
	# Called a(false)
	Result: x = false

Calculating: y = a(false) || b(false)
	# Called a(false)
	# Called b(false)
	Result: y = false

[edit] Liberty BASIC

LB does not have short-circuit evaluation. Implemented with IFs.

print "AND"
for i = 0 to 1
for j = 0 to 1
print "a("; i; ") AND b( "; j; ")"
res =a( i) 'call always
if res <>0 then 'short circuit if 0
res = b( j)
end if
print "=>",res
next
next
 
print "---------------------------------"
print "OR"
for i = 0 to 1
for j = 0 to 1
print "a("; i; ") AND b("; j; ")"
res =a( i) 'call always
if res = 0 then 'short circuit if <>0
res = b( j)
end if
print "=>", res
next
next
 
'----------------------------------------
function a( t)
print ,"calls func a"
a = t
end function
 
function b( t)
print ,"calls func b"
b = t
end function
Output:
AND
a(0) AND b(0)
             calls func a
=>            0
a(0) AND b(1)
             calls func a
=>            0
a(1) AND b(0)
             calls func a
             calls func b
=>            0
a(1) AND b(1)
             calls func a
             calls func b
=>            1
---------------------------------
OR
a(0) AND b(0)
             calls func a
             calls func b
=>            0
a(0) AND b(1)
             calls func a
             calls func b
=>            1
a(1) AND b(0)
             calls func a
=>            1
a(1) AND b(1)
             calls func a
=>            1

[edit]

The AND and OR predicates may take either expressions which are all evaluated beforehand, or lists which are short-circuit evaluated from left to right only until the overall value of the expression can be determined.

and [notequal? :x 0] [1/:x > 3]
(or [:x < 0] [:y < 0] [sqrt :x + sqrt :y < 3])

[edit] Lua

function a(i)
print "Function a(i) called."
return i
end
 
function b(i)
print "Function b(i) called."
return i
end
 
i = true
x = a(i) and b(i); print ""
y = a(i) or b(i); print ""
 
i = false
x = a(i) and b(i); print ""
y = a(i) or b(i)

[edit] Mathematica

Mathematica has built-in short-circuit evaluation of logical expressions.

a[in_] := (Print["a"]; in)
b[in_] := (Print["b"]; in)
 
a[False] && b[True]
a[True] || b[False]

Evaluation of the preceding code gives:

a
False

a
True

Whereas evaluating this:

a[True] && b[False]

Gives:

a
b
False

[edit] MATLAB / Octave

Short-circuit evalation is done in logical AND (&&) and logical OR (||) operators:

  function x=a(x)
printf('a: %i\n',x);
end;
function x=b(x)
printf('b: %i\n',x);
end;
 
a(1) && b(1)
a(0) && b(1)
a(1) || b(1)
a(0) || b(1)
Output:
  > a(1) && b(1);
a: 1
b: 1
> a(0) && b(1);
a: 0
> a(1) || b(1);
a: 1
> a(0) || b(1);
a: 0
b: 1

[edit] MUMPS

MUMPS evaluates every expression it encounters, so we have to use conditional statements to do a short circuiting of the expensive second task.

SSEVAL1(IN)
WRITE !,?10,$STACK($STACK,"PLACE")
QUIT IN
SSEVAL2(IN)
WRITE !,?10,$STACK($STACK,"PLACE")
QUIT IN
SSEVAL3
NEW Z
WRITE "1 AND 1"
SET Z=$$SSEVAL1(1) SET:Z Z=Z&$$SSEVAL2(1)
WRITE !,$SELECT(Z:"TRUE",1:"FALSE")
WRITE !!,"0 AND 1"
SET Z=$$SSEVAL1(0) SET:Z Z=Z&$$SSEVAL2(1)
WRITE !,$SELECT(Z:"TRUE",1:"FALSE")
WRITE !!,"1 OR 1"
SET Z=$$SSEVAL1(1) SET:'Z Z=Z!$$SSEVAL2(1)
WRITE !,$SELECT(Z:"TRUE",1:"FALSE")
WRITE !!,"0 OR 1"
SET Z=$$SSEVAL1(0) SET:'Z Z=Z!$$SSEVAL2(1)
WRITE !,$SELECT(Z:"TRUE",1:"FALSE")
KILL Z
QUIT
Output:
USER>D SSEVAL3^ROSETTA
1 AND 1
          SSEVAL1+1^ROSETTA +3
          SSEVAL2+1^ROSETTA +3
TRUE
 
0 AND 1
          SSEVAL1+1^ROSETTA +3
FALSE
 
1 OR 1
          SSEVAL1+1^ROSETTA +3
TRUE
 
0 OR 1
          SSEVAL1+1^ROSETTA +3
          SSEVAL2+1^ROSETTA +3
TRUE

[edit] Nemerle

using System.Console;
 
class ShortCircuit
{
public static a(x : bool) : bool
{
WriteLine("a");
x
}
 
public static b(x : bool) : bool
{
WriteLine("b");
x
}
 
public static Main() : void
{
def t = true;
def f = false;
 
WriteLine("True && True : {0}", a(t) && b(t));
WriteLine("True && False: {0}", a(t) && b(f));
WriteLine("False && True : {0}", a(f) && b(t));
WriteLine("False && False: {0}", a(f) && b(f));
WriteLine("True || True : {0}", a(t) || b(t));
WriteLine("True || False: {0}", a(t) || b(f));
WriteLine("False || True : {0}", a(f) || b(t));
WriteLine("False || False: {0}", a(f) || b(f));
}
}
Output:
a
b
True && True : True
a
b
True && False: False
a
False && True : False
a
False && False: False
a
True || True : True
a
True || False: True
a
b
False || True : True
a
b
False || False: False

[edit] Objeck

In Objeck the Boolean operators & and | short circuit.

class ShortCircuit {
function : a(a : Bool) ~ Bool {
"a"->PrintLine();
return a;
}
 
function : b(b : Bool) ~ Bool {
"b"->PrintLine();
return b;
}
 
function : Main(args : String[]) ~ Nil {
result := a(false) & b(false);
"F and F = {$result}"->PrintLine();
result := a(false) | b(false);
"F or F = {$result}"->PrintLine();
 
result := a(false) & b(true);
"F and T = {$result}"->PrintLine();
result := a(false) | b(true);
"F or T = {$result}"->PrintLine();
 
result := a(true) & b(false);
"T and F = {$result}"->PrintLine();
result := a(true) | b(false);
"T or F = {$result}"->PrintLine();
 
result := a(true) & b(true);
"T and T = {$result}"->PrintLine();
result := a(true) | b(true);
"T or T = {$result}"->PrintLine();
}
}

[edit] OCaml

let a r = print_endline " > function a called"; r
let b r = print_endline " > function b called"; r
 
let test_and b1 b2 =
Printf.printf "# testing (%b && %b)\n" b1 b2;
ignore (a b1 && b b2)
 
let test_or b1 b2 =
Printf.printf "# testing (%b || %b)\n" b1 b2;
ignore (a b1 || b b2)
 
let test_this test =
test true true;
test true false;
test false true;
test false false;
;;
 
let () =
print_endline "==== Testing and ====";
test_this test_and;
print_endline "==== Testing or ====";
test_this test_or;
;;
Output:
==== Testing and ====
# testing (true && true)
 > function a called
 > function b called
# testing (true && false)
 > function a called
 > function b called
# testing (false && true)
 > function a called
# testing (false && false)
 > function a called
==== Testing or ====
# testing (true || true)
 > function a called
# testing (true || false)
 > function a called
# testing (false || true)
 > function a called
 > function b called
# testing (false || false)
 > function a called
 > function b called

[edit] Oz

Oz' andthen and orelse operators are short-circuiting, as indicated by their name. The library functions Bool.and and Bool.or are not short-circuiting, on the other hand.

declare
fun {A Answer}
AnswerS = {Value.toVirtualString Answer 1 1}
in
{System.showInfo "  % Called function {A "#AnswerS#"} -> "#AnswerS}
Answer
end
 
fun {B Answer}
AnswerS = {Value.toVirtualString Answer 1 1}
in
{System.showInfo "  % Called function {B "#AnswerS#"} -> "#AnswerS}
Answer
end
in
for I in [false true] do
for J in [false true] do
X Y
in
{System.showInfo "\nCalculating: X = {A I} andthen {B J}"}
X = {A I} andthen {B J}
{System.showInfo "Calculating: Y = {A I} orelse {B J}"}
Y = {A I} orelse {B J}
end
end
Output:
Calculating: X = {A I} andthen {B J}
% Called function {A false} -> false
Calculating: Y = {A I} orelse {B J}
% Called function {A false} -> false
% Called function {B false} -> false
 
Calculating: X = {A I} andthen {B J}
% Called function {A false} -> false
Calculating: Y = {A I} orelse {B J}
% Called function {A false} -> false
% Called function {B true} -> true
 
Calculating: X = {A I} andthen {B J}
% Called function {A true} -> true
% Called function {B false} -> false
Calculating: Y = {A I} orelse {B J}
% Called function {A true} -> true
 
Calculating: X = {A I} andthen {B J}
% Called function {A true} -> true
% Called function {B true} -> true
Calculating: Y = {A I} orelse {B J}
% Called function {A true} -> true

[edit] PARI/GP

Note that | and & are deprecated versions of the GP short-circuit operators.

a(n)={
print(a"("n")");
a
};
b(n)={
print("b("n")");
n
};
or(A,B)={
a(A) || b(B)
};
and(A,B)={
a(A) && b(B)
};

[edit] Pascal

[edit] Standard Pascal

Standard Pascal doesn't have native short-circuit evaluation.

program shortcircuit(output);
 
function a(value: boolean): boolean;
begin
writeln('a(', value, ')');
a := value
end;
 
function b(value:boolean): boolean;
begin
writeln('b(', value, ')');
b := value
end;
 
procedure scandor(value1, value2: boolean);
var
result: boolean;
begin
{and}
if a(value1)
then
result := b(value2)
else
result := false;
writeln(value1, ' and ', value2, ' = ', result);
 
{or}
if a(value1)
then
result := true
else
result := b(value2)
writeln(value1, ' or ', value2, ' = ', result);
end;
 
begin
scandor(false, false);
scandor(false, true);
scandor(true, false);
scandor(true, true);
end.

[edit] Turbo Pascal

Turbo Pascal allows short circuit evaluation with a compiler switch:

program shortcircuit;
 
function a(value: boolean): boolean;
begin
writeln('a(', value, ')');
a := value;
end;
 
function b(value:boolean): boolean;
begin
writeln('b(', value, ')');
b := value;
end;
 
{$B-} {enable short circuit evaluation}
procedure scandor(value1, value2: boolean);
var
result: boolean;
begin
result := a(value1) and b(value);
writeln(value1, ' and ', value2, ' = ', result);
 
result := a(value1) or b(value2);
writeln(value1, ' or ', value2, ' = ', result);
end;
 
begin
scandor(false, false);
scandor(false, true);
scandor(true, false);
scandor(true, true);
end.

[edit] Extended Pascal

The extended Pascal standard introduces the operators and_then and or_else for short-circuit evaluation.

program shortcircuit(output);
 
function a(value: boolean): boolean;
begin
writeln('a(', value, ')');
a := value
end;
 
function b(value:boolean): boolean;
begin
writeln('b(', value, ')');
b := value
end;
 
procedure scandor(value1, value2: boolean);
var
result: integer;
begin
result := a(value1) and_then b(value)
writeln(value1, ' and ', value2, ' = ', result);
 
result := a(value1) or_else b(value2);
writeln(value1, ' or ', value2, ' = ', result)
end;
 
begin
scandor(false, false);
scandor(false, true);
scandor(true, false);
scandor(true, true);
end.

Note: GNU Pascal allows and then and or else as alternatives to and_then and or_else.

[edit] Perl

Perl uses short-circuit boolean evaluation.

sub a { print 'A'; return $_[0] }
sub b { print 'B'; return $_[0] }
 
# Test-driver
sub test {
for my $op ('&&','||') {
for (qw(1,1 1,0 0,1 0,0)) {
my ($x,$y) = /(.),(.)/;
print my $str = "a($x) $op b($y)", ': ';
eval $str; print "\n"; } }
}
 
# Test and display
test();
Output:
a(1) && b(1): AB
a(1) && b(0): AB
a(0) && b(1): A
a(0) && b(0): A
a(1) || b(1): A
a(1) || b(0): A
a(0) || b(1): AB
a(0) || b(0): AB

[edit] Perl 6

sub a ($p) { print 'a'; $p }
sub b ($p) { print 'b'; $p }
 
for '&&', '||' -> $op {
for True, False X True, False -> $p, $q {
my $s = "a($p) $op b($q)";
print "$s: ";
eval $s;
print "\n";
}
}
Output:
a(1) && b(1): ab
a(1) && b(0): ab
a(0) && b(1): a
a(0) && b(0): a
a(1) || b(1): a
a(1) || b(0): a
a(0) || b(1): ab
a(0) || b(0): ab

[edit] PicoLisp

(de a (F)
(msg 'a)
F )
 
(de b (F)
(msg 'b)
F )
 
(mapc
'((I J)
(for Op '(and or)
(println I Op J '-> (Op (a I) (b J))) ) )
'(NIL NIL T T)
'(NIL T NIL T) )
Output:
a
NIL and NIL -> NIL
a
b
NIL or NIL -> NIL
a
NIL and T -> NIL
a
b
NIL or T -> T
a
b
T and NIL -> NIL
a
T or NIL -> T
a
b
T and T -> T
a
T or T -> T

[edit] Pike

int(0..1) a(int(0..1) i)
{
write(" a\n");
return i;
}
 
int(0..1) b(int(0..1) i)
{
write(" b\n");
return i;
}
 
foreach(({ ({ false, false }), ({ false, true }), ({ true, true }), ({ true, false }) });; array(int) args)
{
write(" %d && %d\n", @args);
a(args[0]) && b(args[1]);
 
write(" %d || %d\n", @args);
a(args[0]) || b(args[1]);
}
Output:
 0 && 0
 a
 0 || 0
 a
 b
 0 && 1
 a
 0 || 1
 a
 b
 1 && 1
 a
 b
 1 || 1
 a
 1 && 0
 a
 b
 1 || 0
 a

[edit] PL/I

short_circuit_evaluation:
procedure options (main);
declare (true initial ('1'b), false initial ('0'b) ) bit (1);
declare (i, j, x, y) bit (1);
 
a: procedure (bv) returns (bit(1));
declare bv bit(1);
put ('Procedure ' || procedurename() || ' called.');
return (bv);
end a;
b: procedure (bv) returns (bit(1));
declare bv bit(1);
put ('Procedure ' || procedurename() || ' called.');
return (bv);
end b;
 
do i = true, false;
do j = true, false;
put skip(2) list ('Evaluating x with <a> with ' || i || ' and <b> with ' || j);
put skip;
if a(i) then
x = b(j);
else
x = false;
put skip data (x);
put skip(2) list ('Evaluating y with <a> with ' || i || ' and <b> with ' || j);
put skip;
if a(i) then
y = true;
else
y = b(j);
put skip data (y);
end;
end;
end short_circuit_evaluation;
Results:
Evaluating x with <a> with 1 and <b> with 1 
Procedure A called.     Procedure B called. 
X='1'B;

Evaluating y with <a> with 1 and <b> with 1 
Procedure A called. 
Y='1'B;

Evaluating x with <a> with 1 and <b> with 0 
Procedure A called.     Procedure B called. 
X='0'B;

Evaluating y with <a> with 1 and <b> with 0 
Procedure A called. 
Y='1'B;

Evaluating x with <a> with 0 and <b> with 1 
Procedure A called. 
X='0'B;

Evaluating y with <a> with 0 and <b> with 1 
Procedure A called.     Procedure B called. 
Y='1'B;

Evaluating x with <a> with 0 and <b> with 0 
Procedure A called. 
X='0'B;

Evaluating y with <a> with 0 and <b> with 0 
Procedure A called.     Procedure B called. 
Y='0'B;

[edit] Prolog

Prolog has not functions but predicats succeed of fail. Tested with SWI-Prolog. Should work with other dialects.

short_circuit :-
( a_or_b(true, true) -> writeln('==> true'); writeln('==> false')) , nl,
( a_or_b(true, false)-> writeln('==> true'); writeln('==> false')) , nl,
( a_or_b(false, true)-> writeln('==> true'); writeln('==> false')) , nl,
( a_or_b(false, false)-> writeln('==> true'); writeln('==> false')) , nl,
( a_and_b(true, true)-> writeln('==> true'); writeln('==> false')) , nl,
( a_and_b(true, false)-> writeln('==> true'); writeln('==> false')) , nl,
( a_and_b(false, true)-> writeln('==> true'); writeln('==> false')) , nl,
( a_and_b(false, false)-> writeln('==> true'); writeln('==> false')) .
 
 
a_and_b(X, Y) :-
format('a(~w) and b(~w)~n', [X, Y]),
( a(X), b(Y)).
 
a_or_b(X, Y) :-
format('a(~w) or b(~w)~n', [X, Y]),
( a(X); b(Y)).
 
a(X) :-
format('a(~w)~n', [X]),
X.
 
b(X) :-
format('b(~w)~n', [X]),
X.
Output:
?- short_circuit.
a(true) or b(true)
a(true)
==> true
 
a(true) or b(false)
a(true)
==> true
 
a(false) or b(true)
a(false)
b(true)
==> true
 
a(false) or b(false)
a(false)
b(false)
==> false
 
a(true) and b(true)
a(true)
b(true)
==> true
 
a(true) and b(false)
a(true)
b(false)
==> false
 
a(false) and b(true)
a(false)
==> false
 
a(false) and b(false)
a(false)
==> false
 
true.

[edit] PureBasic

Logical And & Or operators will not evaluate their right-hand expression if the outcome can be determined from the value of the left-hand expression.

Procedure a(arg)
PrintN(" # Called function a("+Str(arg)+")")
ProcedureReturn arg
EndProcedure
 
Procedure b(arg)
PrintN(" # Called function b("+Str(arg)+")")
ProcedureReturn arg
EndProcedure
 
OpenConsole()
For a=#False To #True
For b=#False To #True
PrintN(#CRLF$+"Calculating: x = a("+Str(a)+") And b("+Str(b)+")")
x= a(a) And b(b)
PrintN("Calculating: x = a("+Str(a)+") Or b("+Str(b)+")")
y= a(a) Or b(b)
Next
Next
Input()
Output:
Calculating: x = a(0) And b(0)
  # Called function a(0)
Calculating: x = a(0) Or b(0)
  # Called function a(0)
  # Called function b(0)

Calculating: x = a(0) And b(1)
  # Called function a(0)
Calculating: x = a(0) Or b(1)
  # Called function a(0)
  # Called function b(1)

Calculating: x = a(1) And b(0)
  # Called function a(1)
  # Called function b(0)
Calculating: x = a(1) Or b(0)
  # Called function a(1)

Calculating: x = a(1) And b(1)
  # Called function a(1)
  # Called function b(1)
Calculating: x = a(1) Or b(1)
  # Called function a(1)

[edit] Python

Pythons and and or binary, infix, boolean operators will not evaluate their right-hand expression if the outcome can be determined from the value of the left-hand expression.

>>> def a(answer):
print(" # Called function a(%r) -> %r" % (answer, answer))
return answer
 
>>> def b(answer):
print(" # Called function b(%r) -> %r" % (answer, answer))
return answer
 
>>> for i in (False, True):
for j in (False, True):
print ("\nCalculating: x = a(i) and b(j)")
x = a(i) and b(j)
print ("Calculating: y = a(i) or b(j)")
y = a(i) or b(j)
 
 
 
Calculating: x = a(i) and b(j)
# Called function a(False) -> False
Calculating: y = a(i) or b(j)
# Called function a(False) -> False
# Called function b(False) -> False
 
Calculating: x = a(i) and b(j)
# Called function a(False) -> False
Calculating: y = a(i) or b(j)
# Called function a(False) -> False
# Called function b(True) -> True
 
Calculating: x = a(i) and b(j)
# Called function a(True) -> True
# Called function b(False) -> False
Calculating: y = a(i) or b(j)
# Called function a(True) -> True
 
Calculating: x = a(i) and b(j)
# Called function a(True) -> True
# Called function b(True) -> True
Calculating: y = a(i) or b(j)
# Called function a(True) -> True

Pythons if expression can also be used to the same ends (but probably should not):

>>> for i in (False, True):
for j in (False, True):
print ("\nCalculating: x = a(i) and b(j) using x = b(j) if a(i) else False")
x = b(j) if a(i) else False
print ("Calculating: y = a(i) or b(j) using y = b(j) if not a(i) else True")
y = b(j) if not a(i) else True
 
 
 
Calculating: x = a(i) and b(j) using x = b(j) if a(i) else False
# Called function a(False) -> False
Calculating: y = a(i) or b(j) using y = b(j) if not a(i) else True
# Called function a(False) -> False
# Called function b(False) -> False
 
Calculating: x = a(i) and b(j) using x = b(j) if a(i) else False
# Called function a(False) -> False
Calculating: y = a(i) or b(j) using y = b(j) if not a(i) else True
# Called function a(False) -> False
# Called function b(True) -> True
 
Calculating: x = a(i) and b(j) using x = b(j) if a(i) else False
# Called function a(True) -> True
# Called function b(False) -> False
Calculating: y = a(i) or b(j) using y = b(j) if not a(i) else True
# Called function a(True) -> True
 
Calculating: x = a(i) and b(j) using x = b(j) if a(i) else False
# Called function a(True) -> True
# Called function b(True) -> True
Calculating: y = a(i) or b(j) using y = b(j) if not a(i) else True
# Called function a(True) -> True

[edit] R

The builtins && and || will short circuit:

Translation of: Perl
a <- function(x) {cat("a called\n"); x}
b <- function(x) {cat("b called\n"); x}
 
tests <- expand.grid(op=list(quote(`||`), quote(`&&`)), x=c(1,0), y=c(1,0))
 
invisible(apply(tests, 1, function(row) {
call <- substitute(op(a(x),b(y)), row)
cat(deparse(call), "->", eval(call), "\n\n")
}))
Output:
a called
a(1) || b(1) -> TRUE
 
a called
b called
a(1) && b(1) -> TRUE
 
a called
b called
a(0) || b(1) -> TRUE
 
a called
a(0) && b(1) -> FALSE
 
a called
a(1) || b(0) -> TRUE
 
a called
b called
a(1) && b(0) -> FALSE
 
a called
b called
a(0) || b(0) -> FALSE
 
a called
a(0) && b(0) -> FALSE

Because R waits until function arguments are needed before evaluating them, user-defined functions can also short circuit.

switchop <- function(s, x, y) {
if(s < 0) x || y
else if (s > 0) x && y
else xor(x, y)
}
Output:
> switchop(-1, a(1), b(1))
a called
[1] TRUE
> switchop(1, a(1), b(1))
a called
b called
[1] TRUE
> switchop(1, a(0), b(1))
a called
[1] FALSE
> switchop(0, a(0), b(1))
a called
b called
[1] TRUE

[edit] Racket

#lang racket
(define (a x)
(display (~a "a:" x " "))
x)
 
(define (b x)
(display (~a "b:" x " "))
x)
 
(for* ([x '(#t #f)]
[y '(#t #f)])
(displayln `(and (a ,x) (b ,y)))
(and (a x) (b y))
(newline)
 
(displayln `(or (a ,x) (b ,y)))
(or (a x) (b y))
(newline))
Output:
(and (a #t) (b #t))
a:#t b:#t 
(or (a #t) (b #t))
a:#t 
(and (a #t) (b #f))
a:#t b:#f 
(or (a #t) (b #f))
a:#t 
(and (a #f) (b #t))
a:#f 
(or (a #f) (b #t))
a:#f b:#t 
(and (a #f) (b #f))
a:#f 
(or (a #f) (b #f))
a:#f b:#f 

[edit] REXX

The REXX language doesn't have native short circuits (it's specifically mentioned in the language specifications that short-circuiting isn't supported).

/*REXX programs demonstrates  short-circuit  evaulation testing.        */
 
do i=-2 to 2
x=a(i) & b(i)
y=a(i)
if \y then y=b(i)
say copies('─',30) 'x='||x 'y='y 'i='i
end /*j*/
exit /*stick a fork in it, we're done.*/
/*──────────────────────────────────subroutines─────────────────────────*/
a: say 'A entered with:' arg(1);return abs(arg(1)//2) /*1=odd, 0=even */
b: say 'B entered with:' arg(1);return arg(1)<0 /*1=neg, 0=if not*/
Output:
B entered with: -2
A entered with: -2
A entered with: -2
B entered with: -2
────────────────────────────── x=0 y=1 i=-2
B entered with: -1
A entered with: -1
A entered with: -1
────────────────────────────── x=1 y=1 i=-1
B entered with: 0
A entered with: 0
A entered with: 0
B entered with: 0
────────────────────────────── x=0 y=0 i=0
B entered with: 1
A entered with: 1
A entered with: 1
────────────────────────────── x=0 y=1 i=1
B entered with: 2
A entered with: 2
A entered with: 2
B entered with: 2
────────────────────────────── x=0 y=0 i=2

[edit] Ruby

Binary operators are short-circuiting. Demonstration code:

def a( bool )
puts "a( #{bool} ) called"
bool
end
 
def b( bool )
puts "b( #{bool} ) called"
bool
end
 
[true, false].each do |a_val|
[true, false].each do |b_val|
puts "a( #{a_val} ) and b( #{b_val} ) is #{a( a_val ) and b( b_val )}."
puts
puts "a( #{a_val} ) or b( #{b_val} ) is #{a( a_val) or b( b_val )}."
puts
end
end
Output:
a( true ) called
b( true ) called
a( true ) and b( true ) is true.

a( true ) called
a( true ) or b( true ) is true.

a( true ) called
b( false ) called
a( true ) and b( false ) is false.

a( true ) called
a( true ) or b( false ) is true.

a( false ) called
a( false ) and b( true ) is false.

a( false ) called
b( true ) called
a( false ) or b( true ) is true.

a( false ) called
a( false ) and b( false ) is false.

a( false ) called
b( false ) called
a( false ) or b( false ) is false.

[edit] Run BASIC

for k = 1 to 2
ao$ = word$("AND,OR",k,",")
print "========= ";ao$;" =============="
for i = 0 to 1
for j = 0 to 1
print "a("; i; ") ";ao$;" b("; j; ")"
res =a(i) 'call always
'print res;"<===="
if ao$ = "AND" and res <> 0 then res = b(j)
if ao$ = "OR" and res = 0 then res = b(j)
next
next
next k
end
 
function a( t)
print chr$(9);"calls func a"
a = t
end function
 
function b( t)
print chr$(9);"calls func b"
b = t
end function
========= AND ==============
a(0) AND b(0)
	calls func a
a(0) AND b(1)
	calls func a
a(1) AND b(0)
	calls func a
	calls func b
a(1) AND b(1)
	calls func a
	calls func b
========= OR ==============
a(0) OR b(0)
	calls func a
	calls func b
a(0) OR b(1)
	calls func a
	calls func b
a(1) OR b(0)
	calls func a
a(1) OR b(1)
	calls func a

[edit] Sather

class MAIN is
a(v:BOOL):BOOL is
#OUT + "executing a\n";
return v;
end;
b(v:BOOL):BOOL is
#OUT + "executing b\n";
return v;
end;
 
main is
x:BOOL;
 
x := a(false) and b(true);
#OUT + "F and T = " + x + "\n\n";
 
x := a(true) or b(true);
#OUT + "T or T = " + x + "\n\n";
 
x := a(true) and b(false);
#OUT + "T and T = " + x + "\n\n";
 
x := a(false) or b(true);
#OUT + "F or T = " + x + "\n\n";
end;
end;

[edit] Scala

object ShortCircuit {
def a(b:Boolean)={print("Called A=%5b".format(b));b}
def b(b:Boolean)={print(" -> B=%5b".format(b));b}
 
def main(args: Array[String]): Unit = {
val boolVals=List(false,true)
for(aa<-boolVals; bb<-boolVals){
print("\nTesting A=%5b AND B=%5b -> ".format(aa, bb))
a(aa) && b(bb)
}
for(aa<-boolVals; bb<-boolVals){
print("\nTesting A=%5b OR B=%5b -> ".format(aa, bb))
a(aa) || b(bb)
}
println
}
}
Output:
Testing A=false AND B=false   -> Called A=false
Testing A=false AND B= true   -> Called A=false
Testing A= true AND B=false   -> Called A= true -> B=false
Testing A= true AND B= true   -> Called A= true -> B= true
Testing A=false  OR B=false   -> Called A=false -> B=false
Testing A=false  OR B= true   -> Called A=false -> B= true
Testing A= true  OR B=false   -> Called A= true
Testing A= true  OR B= true   -> Called A= true

[edit] Scheme

>(define (a x)
(display "a\n")
x)
>(define (b x)
(display "b\n")
x)
>(for-each (lambda (i)
(for-each (lambda (j)
(display i) (display " and ") (display j) (newline)
(and (a i) (b j))
(display i) (display " or ") (display j) (newline)
(or (a i) (b j))
) '(#t #f))
) '(#t #f))
#t and #t
a
b
#t or #t
a
#t and #f
a
b
#t or #f
a
#f and #t
a
#f or #t
a
b
#f and #f
a
#f or #f
a
b
 

[edit] Seed7

$ include "seed7_05.s7i";
 
const func boolean: a (in boolean: aBool) is func
result
var boolean: result is FALSE;
begin
writeln("a");
result := aBool;
end func;
 
const func boolean: b (in boolean: aBool) is func
result
var boolean: result is FALSE;
begin
writeln("b");
result := aBool;
end func;
 
const proc: test (in boolean: param1, in boolean: param2) is func
begin
writeln(param1 <& " and " <& param2 <& " = " <& a(param1) and b(param2));
writeln(param1 <& " or " <& param2 <& " = " <& a(param1) or b(param2));
end func;
 
const proc: main is func
begin
test(FALSE, FALSE);
test(FALSE, TRUE);
test(TRUE, FALSE);
test(TRUE, TRUE);
end func;
Output:
a
FALSE and FALSE = FALSE
a
b
FALSE or FALSE = FALSE
a
FALSE and TRUE = FALSE
a
b
FALSE or TRUE = TRUE
a
b
TRUE and FALSE = FALSE
a
TRUE or FALSE = TRUE
a
b
TRUE and TRUE = TRUE
a
TRUE or TRUE = TRUE

[edit] Smalltalk

Works with: GNU Smalltalk

The and: or: selectors are shortcircuit selectors but in order to avoid evaluation of the second operand, it must be a block: a and: [ code ] will evaluate the code only if a is true. On the other hand, a and: b, where b is an expression (not a block), behaves like the non-shortcircuit and (&). (Same speech for or |)

Smalltalk at: #a put: nil.
Smalltalk at: #b put: nil.
 
a := [:x| 'executing a' displayNl. x].
b := [:x| 'executing b' displayNl. x].
 
('false and false = %1' %
{ (a value: false) and: [ b value: false ] })
displayNl.
 
('true or false = %1' %
{ (a value: true) or: [ b value: false ] })
displayNl.
 
('false or true = %1' %
{ (a value: false) or: [ b value: true ] })
displayNl.
 
('true and false = %1' %
{ (a value: true) and: [ b value: false ] })
displayNl.

[edit] SNOBOL4

Because of its unique success/failure model of flow control, Snobol does not use standard boolean operators or assignment. However, in &fullscan mode Snobol exhibits short-circuit boolean behavior in pattern matches, with concatenation " " functioning as logical AND, and alternation " | " as logical OR.

The test statements below use a pattern constructed from the functions a( ) and b( ) and match it to the null string with deferred evaluation. This idiom allows the functions to self-report the expected short-circuit patterns.

        define('a(val)') :(a_end)
a out = 'A '
eq(val,1) :s(return)f(freturn)
a_end
 
define('b(val)') :(b_end)
b out = 'B '
eq(val,1) :s(return)f(freturn)
b_end
 
* # Test and display
&fullscan = 1
output(.out,1,'-[-r1]') ;* Macro Spitbol
* output(.out,1,'B','-')  ;* CSnobol
define('nl()'):(nlx);nl output = :(return);nlx
 
out = 'T and T: '; null ? *a(1) *b(1); nl()
out = 'T and F: '; null ? *a(1) *b(0); nl()
out = 'F and T: '; null ? *a(0) *b(1); nl()
out = 'F and F: '; null ? *a(0) *b(0); nl()
output =
out = 'T or T: '; null ? *a(1) | *b(1); nl()
out = 'T or F: '; null ? *a(1) | *b(0); nl()
out = 'F or T: '; null ? *a(0) | *b(1); nl()
out = 'F or F: '; null ? *a(0) | *b(0); nl()
end
Output:
T and T: A B
T and F: A B
F and T: A
F and F: A

T or T: A
T or F: A
F or T: A B
F or F: A B

[edit] Standard ML

Translation of: OCaml
fun a r = ( print " > function a called\n"; r )
fun b r = ( print " > function b called\n"; r )
 
fun test_and b1 b2 = (
print ("# testing (" ^ Bool.toString b1 ^ " andalso " ^ Bool.toString b2 ^ ")\n");
ignore (a b1 andalso b b2) )
 
fun test_or b1 b2 = (
print ("# testing (" ^ Bool.toString b1 ^ " orelse " ^ Bool.toString b2 ^ ")\n");
ignore (a b1 orelse b b2) )
 
fun test_this test = (
test true true;
test true false;
test false true;
test false false )
;
 
print "==== Testing and ====\n";
test_this test_and;
print "==== Testing or ====\n";
test_this test_or;
Output:
==== Testing and ====
# testing (true andalso true)
 > function a called
 > function b called
# testing (true andalso false)
 > function a called
 > function b called
# testing (false andalso true)
 > function a called
# testing (false andalso false)
 > function a called
==== Testing or ====
# testing (true orelse true)
 > function a called
# testing (true orelse false)
 > function a called
# testing (false orelse true)
 > function a called
 > function b called
# testing (false orelse false)
 > function a called
 > function b called

[edit] Tcl

The && and || in the expr command support short-circuit evaluation. It is recommended that you always put expressions in braces so that and command or variable substitutions are applied at the right time rather than before the expression is evaluated at all. (Indeed, it is recommended that you do that anyway as unbraced expressions cannot be efficiently compiled.)

package require Tcl 8.5
proc tcl::mathfunc::a boolean {
puts "a($boolean) called"
return $boolean
}
proc tcl::mathfunc::b boolean {
puts "b($boolean) called"
return $boolean
}
 
foreach i {false true} {
foreach j {false true} {
set x [expr {a($i) && b($j)}]
puts "x = a($i) && b($j) = $x"
set y [expr {a($i) || b($j)}]
puts "y = a($i) || b($j) = $y"
puts ""; # Blank line for clarity
}
}
Output:
Note that booleans may be written out words or numeric:
a(false) called
x = a(false) && b(false) = 0
a(false) called
b(false) called
y = a(false) || b(false) = 0

a(false) called
x = a(false) && b(true) = 0
a(false) called
b(true) called
y = a(false) || b(true) = 1

a(true) called
b(false) called
x = a(true) && b(false) = 0
a(true) called
y = a(true) || b(false) = 1

a(true) called
b(true) called
x = a(true) && b(true) = 1
a(true) called
y = a(true) || b(true) = 1

[edit] TXR

@(define a (x out))
@ (output)
a (@x) called
@ (end)
@ (bind out x)
@(end)
@(define b (x out))
@ (output)
b (@x) called
@ (end)
@ (bind out x)
@(end)
@(define short_circuit_demo (i j))
@ (output)
a(@i) and b(@j):
@ (end)
@ (maybe)
@ (a i "1")
@ (b j "1")
@ (end)
@ (output)
a(@i) or b(@j):
@ (end)
@ (cases)
@ (a i "1")
@ (or)
@ (b j "1")
@ (or)
@ (accept)
@ (end)
@(end)
@(short_circuit_demo "0" "0")
@(short_circuit_demo "0" "1")
@(short_circuit_demo "1" "0")
@(short_circuit_demo "1" "1")
Run:
$ txr short-circuit-bool.txr 
a(0) and b(0):
  a (0) called
a(0) or b(0):
  a (0) called
  b (0) called
a(0) and b(1):
  a (0) called
a(0) or b(1):
  a (0) called
  b (1) called
a(1) and b(0):
  a (1) called
  b (0) called
a(1) or b(0):
  a (1) called
a(1) and b(1):
  a (1) called
  b (1) called
a(1) or b(1):
  a (1) called

The a and b functions are defined such that the second parameter is intended to be an unbound variable. When the function binds out, that value propagates back to the unbound variable at the call site. But the way calls works in this language allows us to specify a value instead such as "1". So now the directive @(bind out x) performs unification instead: if x doesn't match "1", the function fails, otherwise it succeeds.

So simply by placing two calls consecutively, we get a short circuting conjunction. The second will not execute if the first one fails.

Short-circuiting disjunction is provided by @(cases).

The @(maybe) construct stops failure from propagating from the enclosed subquery. The @(accept) directive will bail out of the closest enclosing anonymous block (the function body) with a success. It prevents the @(cases) from failing the function if neither case is successful.

[edit] UNIX Shell

The && and || operators use the exit status of each command. The true and false commands convert a string to an exit status; our code && x=true || x=false converts an exit status to a string.

Works with: Bourne Shell
a() {
echo "Called a $1"
"$1"
}
 
b() {
echo "Called b $1"
"$1"
}
 
for i in false true; do
for j in false true; do
a $i && b $j && x=true || x=false
echo " $i && $j is $x"
 
a $i || b $j && y=true || y=false
echo " $i || $j is $y"
done
done

The output reveals that && and || have short-circuit evaluation.

Called a false
  false && false is false
Called a false
Called b false
  false || false is false
Called a false
  false && true is false
Called a false
Called b true
  false || true is true
Called a true
Called b false
  true && false is false
Called a true
  true || false is true
Called a true
Called b true
  true && true is true
Called a true
  true || true is true

[edit] C Shell

Between commands, && and || have short-circuit evaluation. (The aliases for a and b must expand to a single command; these aliases expand to an eval command.)

alias a eval \''echo "Called a \!:1"; "\!:1"'\'
alias b eval \''echo "Called b \!:1"; "\!:1"'\'
 
foreach i (false true)
foreach j (false true)
a $i && b $j && set x=true || set x=false
echo " $i && $j is $x"
 
a $i || b $j && set x=true || set x=false
echo " $i || $j is $x"
end
end

Inside expressions, && and || can short circuit some commands, but cannot prevent substitutions.

# Succeeds, only prints "ok".
if ( 1 || { echo This command never runs. } ) echo ok
 
# Fails, aborts shell with "bad: Undefined variable".
if ( 1 || $bad ) echo ok
 
# Prints "error", then "ok".
if ( 1 || `echo error >/dev/stderr` ) echo ok

[edit] zkl

fcn a(b){self.fcn.println(b); b}
fcn b(b){self.fcn.println(b); b}
Output:
a(True)  or b(True)  //-->Fcn(a)True, True
a(False) or b(True)  //-->Fcn(a)False, Fcn(b)True, True
a(False) or b(False) //-->Fcn(a)False, Fcn(b)False, False

a(True)  and b(True)  //-->Fcn(a)True, Fcn(b)True, True
a(True)  and b(False) //-->Fcn(a)True, Fcn(b)False, False
a(False) and b(True)  //-->Fcn(a)False, False
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